The Hawaiian Islands are an unequaled classroom in the study of evolutionary diversity. Because of their extreme isolation from any other land mass, Hawaii's flora evolved slowly over millions of years. Many plants diverged significantly from their founding ancestors, which arrived from elsewhere against great odds. The result is a spectacular example of adaptive radiation, which has produced new and fantastic forms of plant life. Because of the great diversity of life zones and endemic species, it takes a little time to learn identify Hawaii's flora, but the effort produces rich rewards for the curious and determined.
Note the elevation and region where the plant is growing. Hawaiian plant communities grow in particular geographic regions determined largely by elevation and their relative position on the island. Observe climatic factors like rainfall, temperature and exposure to salt spray to determine if the plant is a member of a coastal community, dry forest or wet forest.
Note the overall appearance of the plant. In Hawaii, it is quite common for a single genus to take radically different forms, with one species perhaps growing as a vine, and another as a large tree. Examine one of the plant's leaves closely with a hand lens, without removing it from the plant. Make a note of the patterns the veins make and the leaf's overall shape. Check if it is shiny or hairy; toothed or smooth at the edges. Decide if it is single, like an ivy leaf, or compound, like a rose leaf. Observe if the leaves appear opposite each other on the stem, alternate, or in whorls.
Look for fruit, seeds, spores or any other distinguishing characteristics. Determine if the plant is a fern, palm, grass or flowering plant. Study the undersides of leaves for spores, which indicate the plant is a fern. If the plant looks like a grass, check to see if the stem is triangular in the cross section, indicating it is actually a sedge. Take photographs or make sketches of the plant's various parts.
Determine if the plant is native or introduced. Unfortunately in Hawaii, it is almost easier to learn to identify weed plants first, since they are so widespread. Spend time getting to know native plants in National and State Parks, Natural Area Reserves and other protected areas. Learn to recognize common weeds in agricultural areas, roadsides and densely populated urban or coastal locations.
Compare the plant to photographs, drawings and descriptions in field guides or botanical keys. Since Hawaii's ecosystems are so diverse, it would be extremely difficult to create a comprehensive field guide relevant to all areas of the state. Search for books describing plant communities growing in specific locations you can visit, such as guidebooks for particular parks and nature walks.